Rapid and reliable detection of microorganisms in a culture, such as a blood culture, is among the most important functions of the clinical microbiology laboratory. Currently, the presence of biologically active agents such as bacteria in a patient's body fluid, and especially in blood, is determined using culture vials. A small quantity of the patient's body fluid is injected through an enclosing rubber septum into a sterile vial containing a culture medium and the vial is then incubated and monitored for microorganism growth.
Common visual inspection of the culture vial then involves monitoring the turbidity or observing eventual color changes of the liquid suspension within the vial. Known instrument methods can also be used to detect changes in the carbon dioxide content of the culture vessels, which is a metabolic byproduct of the bacterial growth. Monitoring the carbon dioxide content can be accomplished by methods well established in the art.
In some instances, non-invasive infrared microorganism detection instrument is used in which special vials having infrared-transmitting windows are utilized. In some instances, glass vials are transferred to an infrared spectrometer by an automated manipulator arm and measured through the glass vial. In some instances, chemical sensors are disposed inside the vial. These sensors respond to changes in the carbon dioxide concentration in the liquid phase by changing their color or by changing their fluorescence intensity. These techniques are based on light intensity measurements and require spectral filtering in the excitation and/or emission signals.
As the above indicates, several different culture systems and approaches are available to laboratories. For example, the BACTEC® radiometric and nonradiometric systems (Becton Dickenson Diagnostic Instrument Systems, Sparks, Md.) are often used for this task. The BACTEC® 9240 instrument, for example, accommodates up to 240 culture vessels and serves as an incubator, agitator, and detection system. Each vessel contains a fluorescent CO2 sensor, and the sensors are monitored on a continuous basis (e.g., every ten minutes). Cultures are recognized as positive by computer algorithms for growth detection based on an increasing rate of change as well as sustained increase in CO2 production rather than by the use of growth index threshold or delta values. The BACTEC® 9240 is completely automated once the vessels have been loaded.
One drawback with these microorganism detection approaches is that they do not always detect cultures that contain microorganisms. Thus, what are needed in each of the above-identified systems are improved methods for determining whether a culture in a vessel contains a plurality of microorganisms.